In the field of detectors used for infrared imaging or thermography (pyrometry), it is known to use devices configured in the form of an array and capable of operating at ambient temperature which use a variation in a physical unit of a material or an assembly of appropriate materials as a function of temperature at around 300 K, i.e. without cooling. In the particular case of bolometric detectors which are the most widely used, this physical unit is electrical resistivity. Other electrical units such as dielectric constant, polarisation and even non-electrical units such as differential thermal expansion, refractive index, etc. can be exploited. These detectors are generically referred to as thermal detectors, in contrast to (photoconductive or photovoltaic) quantum detectors which only operate properly at extremely low temperatures.
Such a non-cooled detector, in its ready-to-operate state, is usually obtained by assembling the following elements:                a substrate comprising means of matrix addressing the sensitive elements (elementary bolometers) and forming an electrical signal on the basis of each element. This substrate is commonly referred to as a Read-Out Integrated Circuit (ROIC). The surface of the substrate carries a matrix assembly of sensitive structures, each essentially formed by a membrane that is suspended by means of extremely fine, narrow arms;        a usually highly exhausted hermetically sealed enclosure (or package) that has an illuminated face with a window that is transparent to the radiation to be detected and electrical connections designed for the external pins of the device. The “vacuum” is intended to ensure high thermal isolation between the substrate and the sensitive elements. This thermal resistance, which is fundamental in terms of ensuring that the detector is extremely sensitive, is defined by the shape of the suspension arms and the materials from which they are made.        
The substrate, provided with the sensitive elements, is fitted in the cavity inside the package facing the window by bonding or brazing and the electrical connections of the substrate are separately mounted on the internal inputs/outputs of the package by wire bonding. This package also contains:                an electrically or thermally activated getter element designed to maintain an adequate partial vacuum inside the component throughout its service life after the component has been exhausted and hermetically sealed;        a ThermoElectric Cooler (TEC) capable of controlling the temperature of the substrate and which is inserted by bonding or brazing between the case of the package and the substrate. Use of this module is intended to eliminate the effects of temperature variations on the focal plane when the detector is in use; at present, only top-of-the-range components are equipped with such a module.        
In order to obtain a scene using this detector, the scene is projected through suitable optics onto the array of elementary detectors and clocked electrical stimuli are applied via the readout circuit to each of the elementary detectors or to each row of such detectors in order to obtain an analogue and/or digital video type electrical signal that is representative of the temperature reached by each of said elementary detectors and is intended, for instance, to form a thermal image of the observed scene.
Components that are relatively simple compared with the above description are currently fabricated by assembling two parts formed by two different substrates, typically using Wafer Level Packaging (WLP) techniques. Document WO 95/17014 describes an object and a fabrication method of this type. One thus initially obtains the depressurised enclosure containing a thin-film getter, the enclosure is delimited by the two substrate components and a peripheral sealing bead. One of the substrates comprises the readout circuit and the sensitive elements, the other substrate provides the window.
The main attraction of this type of technology is the fact that a large number of hermetically sealed components can be obtained simultaneously by using a relatively limited number of parts and operations, hence saving costs. Interfacing with the external environment of the components after singulation by mechanically cutting the two substrates is, in principle, obtained in a second stage by integrating the single assembly on a base that uses, for example, Printed Circuit Board (PCB) technology and has standard metallic tracks and may also comprise electronic proximity circuitry.
The overall process of producing the depressurised enclosure, including final cutting which cannot be performed on both substrates simultaneously because the electrical input/output connections formed outside the enclosure on the substrate of the readout circuit have to be exposed, nevertheless remains relatively complex. Using two different substrates also makes it necessary to employ multiple techniques that are specific to each substrate in order to obtain the various characteristics that are necessary for their operation. For the substrate that provides the windows, for example, highly planar cavities that face each sensitive array have to be formed and localised antireflective layers have to be deposited in them. Also, both the substrates must have multilayer metallisation that is provided in order to braze the substrates together. All these techniques can be mastered but they require numerous items of expensive equipment. In addition, the technologies involved in brazing large-area substrates so that all the final components are collectively hermetically sealed have to be mastered, and this imposes particular constraints in terms of the flatness and geometrical quality of the two surfaces that are to be joined. Finally, these operations are carried out when the extremely fragile sensitive elements are exposed on the surface of the readout circuit substrate and this makes the operation especially tricky with regard to the integrity of the structures and the risk of particulate contamination.
Although they undoubtedly represent progress in terms of industrial manufacturing, these techniques nevertheless remain relatively complex and costly.
One way of partly overcoming these limitations is described in document FR 2 822 541. This document describes an object that comprises micro-cavities formed facing each detection site by using collective microelectronic technologies, thus making the fabrication process applicable in order to achieve functional depressurisation. According to the information disclosed in said document, there is no longer any need for a second substrate and this eliminates the inherent difficulties of WLP techniques, limits the number of operations and obviates the need to deploy a wide variety of technologies. What is more, vulnerability of the structures and risk of particulate contamination during fabrication, i.e. the associated reduced yields or the cost of the requisite precautions to prevent reduced yields, can be regarded as being practically zero.
However, producing micro-capsules or micro-cavities involves a loss of space over the entire footprint of each elementary detector, and this has an impact on the ultimate sensitivity of the component due to a reduction in the fill factor which (in simple terms) is related to the surface area of the sensitive membrane compared to surface area p2, where p denotes the repetition pitch of the array of unitary elements (pixels) of the thermal imager. In addition, the structures that anchor the support arms of each membrane must be formed strictly inside the internal surface area delimited by the side walls that separate each micro-capsule from its neighbour. This results in a loss of sensitivity associated with limitations on the practicable length of said arms; this length defines the thermal resistance between the membrane and the substrate. However, thermal resistance is the crucial factor that determines the sensitivity of thermal detectors.
These drawbacks are not particularly detrimental if the pitch of the array is relatively widely spaced, typically down to 35 or 30 μm or even 25 μm. However, there is currently growing demand for imaging arrays with extremely high spatial resolutions having pitches down to 20 μm or even 15 μm, essentially without thereby having to accept any loss in the sensitivity of the elementary bolometer. There is therefore a need to design structures that are capable of achieving such results whilst retaining the manufacturing advantages of previous technologies.
The underlying principle of the information disclosed in the latter document is described in relation to FIGS. 1A and 1B.
FIG. 1A schematically shows a partial plan view of an array of detectors, ignoring the special features provided to allow exhaustion and hermetic sealing of the micro-sites.
The object has:                an initial substrate 1 on which the structures are collectively formed; the surface of this substrate comprises all the necessary electronic elements for the device to operate and there is no need to detail this point here        sensitive membranes 2, laid out in an array with a repetition pitch p and suspended by their support arms 6;        anchoring structures 3 on which the ends of arms 6 rest;        walls or peripheral walls 4 of the micro-capsules which separate the detection micro-sites;        top covers or windows 5 which are essentially transparent and close the micro-capsules by resting on the upper end of walls 4.        
FIG. 1B supplements the description and shows a cross-section along the dotted line in FIG. 1A. There is no need to describe the construction or geometry of these elements in greater detail here but it is appropriate to specify the approximate size and geometry of the assembly.
The space between the substrate and the membranes is typically around 2 μm in order to optimise sensitivity in the usual infrared band between wavelengths of 8 to 14 μm provided a reflector (not shown) is formed on the surface of the substrate, as is well known. The space between membranes 2 and window 5 is also typically around 2 μm in order to prevent the elements being too close to each other once the cavity has been exhausted. The pitch p of such a structure is typically more than 25 μm.
It is apparent that the footprint of anchoring structures 3 (four of them are shown in FIG. 1A and this number may possibly be reduced to two if the mechanical stability of the membranes so permits) substantially limits the radiant energy that each membrane 2 can efficiently collect as a proportion of the total energy received on surface area p2.
In addition, the footprint of walls 4 and the adjacent peripheral space inside the micro-capsules between walls 4 and membranes 2 also partially limit the radiant energy that can be collected by the membrane.
In other words, these geometrical considerations impose substantial limits on the fill factor, especially in case of narrower pitches.
There is another limitation in terms of the length of support arm 6 that can be realised using anchoring structures formed inside the walls compared with structures which do not have micro-capsules but do have anchoring points that are common to two (or even four) adjacent membranes. This advantageous layout, obtained by forming the anchoring points along the axis of symmetry between two (or four) sensitive sites is routine in this field when using conventional technology without micro-capsules and actually sets the level of performance of the known technology.
The aim of the invention is therefore to propose such structures and their fabrication methods in order to ultimately form detectors that benefit from enhanced performance whilst still enjoying the advantages provided by vacuum-tight micro-site technology.
Another aim of the invention is to propose specific devices, as well as their fabrication method, these devices having electro-optical characteristics that are advantageously obtained within the scope of the invention:                detectors which have micro-sites that are sensitive in several spectral bands respectively,        detectors which have locally situated reference micro-sites that are insensitive to infrared radiation,        detectors which have the ability to detect only certain polarisation orientations or directions of incidence of radiation or which have a uniform or distributed pixel high-pass filter with several cut-off wavelengths,        detectors which incorporate protection against intense radiation.        
The invention is mostly detailed in the rest of this description on the basis of its architecture details which are suitable for the most common case and that which is of particular interest—detection in the Long Wave InfraRed (LWIR) region which corresponds to the 8 to 14 μm infrared atmospheric transmission window. Nevertheless, it is possible to adapt the invention's constructional details in order to exploit it in other infrared bands, even beyond the infrared region in the so-called “terahertz” domain. This is why the term “electromagnetic radiation” is sometimes used here in preference to the more limitative term “infrared”.
Also, in the rest of this description, the terms “radiation of interest” or, more precisely, “spectral band of interest”, the limits of which are subsequently denoted by λmin and λmax, are to be construed as the range of wavelengths for which the detector is intended (designed) to be sensitive. In the case of microbolometers, the membrane itself is usually sensitive to a very wide range of radiation frequencies (in other words it is not particularly wavelength sensitive) and, as a result, the “spectral band of interest” is in fact defined by the spectrum over which the window (in this case the window cover) is substantially transparent.
The terms “preferred spectral sensitivity” or “preferred spectrum” are also to be construed as the obtainment of maximum sensitivity (or optical response) of the detector around a particular wavelength λp within the said spectral band of interest by means of a spectral transmission mask of the window cover that exhibits maximum sensitivity in the vicinity of that wavelength.